At present, electronic devices have many integrated circuits in use. The many integrated circuits used in the electronic devices are so-called application-specific integrated circuits (ASIC) that are dedicated circuits designed for use in those electronic devices. After an application-specific integrated circuit has been manufactured, its circuit configurations cannot be changed because logic cells (logic circuits serving as units, such as AND circuits and OR circuits) are laid out and interconnected in the integrated circuit fabrication process.
Recent years have seen intensive electronic device development races and electronic device miniaturization. Under the circumstances, attention has been directed to programmable logic devices (rewritable logic integrated circuits) which are capable of selecting certain functions from many functions on one chip by changing circuit configurations with electric signals even after the programmable logic devices have been manufactured.
A programmable logic device comprises a plurality of logic cells interconnected through switches. Typical examples of programmable logic devices include an FPGA (Field-Programmable Gate array) and a DRP (Dynamically Reconfigurable Processor).
Although programmable logic devices are attracting attention, they have been installed in electronic devices in limited applications thus far. The reasons for their limited use will be described below. In programmable logic devices available up to now, the switches which interconnect logic cells are large in size and high in on-resistance. In order to restrict the installed number of such switches as much as possible, it has been customary to employ a small number of logic cells each comprising many transistors. As a result, the logic cells can be used in limited combinations, resulting in limited functions that the programmable logic device can provide. In other words, the large size and high on-resistance of the switches that are used in the programmable logic devices available up to now have limited the functions of the programmable logic devices and also have limited their installation into electronic devices.
For giving programmable logic devices increased multiple functionality and promoting their installation into electronic devices, it is necessary to reduce the size and on-resistance of switches interconnecting logic cells in the programmable logic devices.
As a switch device which meets the above requirements, a switching device based on the metal ion movement and electrochemical reaction in an ion conductor (a solid substance in which ions can move freely) is disclosed in “Journal of Solid-State Circuits”, Vol. 40, No. 1, pages 168 through 176, 2005 (hereinafter referred to as “Document 1”).
The switching device based on the metal ion movement and electrochemical reaction in an ion conduction layer as disclosed in Document 1 comprises three layers including an ion conduction layer and first and second electrodes which are disposed on opposite surfaces held in contact with the ion conduction layer. The first electrode serves to supply metal ions to the ion conduction layer. The second electrode does not supply metal ions.
Operation of the switching device will briefly be described below. When the first electrode is connected to ground and a negative voltage is applied to the second electrode, metal ions from the metal of the first electrode are dissolved into the ion conduction layer. The metal ions in the ion conduction layer are precipitated as metal in the ion conduction layer, forming a metal bridge which connects the first electrode and the second electrode to each other. When the first electrode and the second electrode are electrically connected to each other by the metal bridge, the switch is turned on.
When the first electrode is connected to ground and a positive voltage is applied to the second electrode while the switch is on, a portion of the metal bridge is cut off. The first electrode and the second electrode are now electrically disconnected from each other, turning off the switch. Specifically, before the first electrode and the second electrode are fully disconnected from each other, the resistance between the first electrode and the second electrode increases and the capacitance therebetween changes, i.e., the electric characteristics of the switch change. After the electric characteristics of the switch have changed, the first electrode and the second electrode are finally disconnected from each other. To turn on the switch that has been turned off, a negative voltage may be applied again to the first electrode.
Document 1 reveals the structure and operation of the two-terminal switch for controlling the conduction between the two electrodes with the ion conductor interposed between. “WO2005/008783 publication” (hereinafter referred to as “Document 2”) proposes a three-terminal ion conductor switching device which additionally includes a control electrode (third electrode) for controlling the conduction between the first and second electrodes based on a voltage applied to the control electrode.
Switching devices including an ion conductor are considered to be promising for use in programmable logic devices because they are smaller in size and lower in on-resistance than semiconductor switches (MOSFETs or the like) that have generally been used. Such a switch is also applicable as a nonvolatile memory device because its conduction state (on or off remains unchanged even when the applied voltage is turned off. For example, a nonvolatile memory is constructed of a plurality of memory cells, each comprising a selective device such as a transistor and a switching device including an ion conductor, which are arrayed in a matrix of vertical columns and horizontal rows. Any desired one of the memory cells is selected by signals supplied to word lines and bit lines. The conduction state of the switching device of the selected memory cells is sensed to read information of either “1” or “0” based on whether the switching device is turned or off (see Document 1).
Methods of manufacturing such a switching device in an integrated circuit are disclosed in U.S. Pat. No. 6,348,365 (hereinafter referred to as “Document 3”) and U.S. Pat. No. 6,838,307 (hereinafter referred to as “Document 4”).
FIG. 1 is a cross-sectional view showing the structure of the switching device disclosed in Document 3. According to the device structure disclosed in Document 3, as shown in FIG. 1, a stacked-layer structure comprising a metal layer (indicated as “metal material 41”) and an ion conductor (indicated as “ion conduction layer 51”) is embedded in an opening in an insulating layer (indicated as “insulating material 13”). The ion conduction layer is made of chalcogenide in the form of a germanium and selenium layer including silver. The structure is fabricated as follows: After the ion conduction layer is embedded in the opening, a recessed structure is formed. The metal layer is embedded in the recessed structure and diffused by the application of light, thereby producing the desired stacked-layer structure. As shown in FIG. 1, the metal layer for supplying metal ions is held in contact with the insulating layer. The insulating layer with the opening is disposed on conductive material 12 disposed on insulating material 11 which is disposed on semiconductor substrate 10.
FIG. 2 is a cross-sectional view showing the structure of the switching device disclosed in Document 4. According to the device structure disclosed in Document 4, upper electrode 133 is partly embedded in an opening in insulating layer 121, and ion conduction layer 107 (referred to as “cell body” in Document 4) is embedded beneath upper electrode 133. Spacer 131 is held against the side wall of the opening in insulating layer 121. Therefore, the upper surface of ion conduction layer 107 is partly held in contact with upper electrode 133. When metal ions are supplied from upper electrode 133 to ion conductor 107, spacer 131 serves to prevent the metal ions from entering the boundary between insulating layer 121 and ion conductor 107. As a result, the metal ions are uniformly supplied to ion conductor 107.